U.S. patent number 4,421,951 [Application Number 06/305,124] was granted by the patent office on 1983-12-20 for method and arrangement for signaling the transmission mode of a communication system.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to John G. Van Bosse.
United States Patent |
4,421,951 |
Van Bosse |
December 20, 1983 |
**Please see images for:
( Certificate of Correction ) ** |
Method and arrangement for signaling the transmission mode of a
communication system
Abstract
The disclosed communication system operating mode signaling
method and arrangement utilizes dc current signaling over the loop
facility connecting a customer to a central office (CO). The dc
current on the facility provides concurrent on-hook/off-hook
signaling and voice/digital data operating mode signaling between
the customer and CO. A current flow in a first direction over the
facility signals a voice operating mode while a current flow in a
second direction signals a digital data operating mode. A customer
signals a change in the operating mode by reversing the direction
of allowed current flow at the customer unit. The resulting
interruption of the current flow is detected at the CO which
responds by reversing the direction of current flow over the
facility. The reappearance of current flow over the facility
distinguishes a customer's change in operating mode from an on-hook
condition.
Inventors: |
Van Bosse; John G. (Naperville,
IL) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
23179437 |
Appl.
No.: |
06/305,124 |
Filed: |
September 24, 1981 |
Current U.S.
Class: |
379/93.09;
375/216; 379/382 |
Current CPC
Class: |
H04M
11/06 (20130101) |
Current International
Class: |
H04M
11/06 (20060101); H04M 011/00 () |
Field of
Search: |
;179/2C,2CA,2DP,3,4
;307/236 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Taley, D.; Basic Telephone Switching Systems, 1969; pp. 26-29,
34-37; Hayden Book Co..
|
Primary Examiner: Rubinson; G. Z.
Assistant Examiner: George; Keith E.
Attorney, Agent or Firm: Caccuro; John A.
Claims
What is claimed is:
1. In a communication system including a subscriber unit adapted
for connection over a transmission facility to a central unit, said
system operating in either an analog or a digital communication
mode, apparatus for signaling the communication mode between said
subscriber unit and said central unit
characterized in that
said apparatus includes
first means for providing at said central unit a first polarity dc
signal over said facility representing a first operating mode,
means for changing at said subscriber unit said first polarity dc
signal received over said facility to signal the central unit of a
change in the communication mode at said subscriber unit, and
second means for providing at said central unit in response to an
interruption of said first polarity dc signal a second polarity dc
signal over said facility representing a second operating mode.
2. An arrangement for signaling the operating modes of a
communication system, said arrangement including a subscriber unit
adapted for connection over a transmission facility to a central
unit
characterized in that
said subscriber unit includes
first means for signaling a first operating mode to said central
unit by allowing the reception of a first polarity dc signal over
said facility,
second means for signaling a second operating mode to said central
unit by allowing the reception of a second polarity dc signal over
said facility,
means for coupling either said first signaling means or said second
signaling means to said facility, and
said central unit includes
first means for providing a first polarity dc signal over said
facility,
second means for providing a second polarity dc signal over said
facility, and
switching means for alternating the connection of said first
providing means and said second providing means to said facility in
response to a dc signal interruption on said facility resulting
from an operation of said coupling means.
3. The invention of claim 1 or claim 2 wherein said central unit
includes
means responsive to a first control signal for connecting either
said first providing means or said second providing means to said
facility,
means for detecting an interruption of a dc signal on said
facility, and
means responsive to said detecting means for producing said first
control signal to said connecting means to interchange the
providing means connected to said facility.
4. The invention of claim 3 wherein said central unit further
includes
means responsive to a signal received over a second facility from a
remote unit for establishing a second control signal to connect
either said first providing means or said second providing means to
said facility, and
means for transferring control of said connecting means from said
first control signal of said producing means to said second control
signal of said establishing means.
5. The invention of claim 1 or claim 2 wherein said central unit
further includes
means for comparing the operating mode of said subscriber unit with
the operating mode of a remote unit connected to said central unit
over a second facility and
means for sending a conflict signal over said facility to said
subscriber unit in response to an output from said comparing means
indicating a difference in operating modes between said subscriber
unit and said remote unit.
6. The invention of claim 1 or claim 2 wherein said central unit
further includes
means responsive to a continued interruption in the dc signal on
said facility for terminating the connection between said
subscriber unit and said central unit.
7. The invention of claim 1 or claim 2 wherein said central unit
further includes
means responsive to a dc signal interruption for transmitting a
signal representing the operating mode of said subscriber unit over
a second facility to a remote unit.
8. Apparatus for controlling the analog or digital communication
mode of a transmission facility
characterized in that
said apparatus includes
means responsive to a control signal for providing either a first
polarity dc signal over said facility to represent a first
communication mode or a second polarity dc signal over said
facility to represent a second communication mode and
control means for generating said control signal to interchange the
existing coupling of said providing means in response to a
predetermined interruption in the dc signal on said facility.
9. The invention of claim 8 wherein said apparatus further
includes
means responsive to a signal received over a second facility from a
remote unit for establishing a second control signal to said
providing means and
means for transferring control of said providing means from said
control signal of said control means to said second control signal
of said establishing means.
10. The invention of claim 8 wherein said apparatus further
includes
means for comparing the communication mode of said subscriber unit
with the communication mode of a remote unit connected to said
apparatus over a second facility and
means for sending a conflict signal over said facility to said
subscriber in response to an output from said comparing means
indicating a difference in communication modes between said
subscriber unit and said remote unit.
11. The invention of claim 9 wherein said conflict signal from said
sending means is a signal indicating the operating mode of said
remote unit.
12. The invention of claim 8 wherein said apparatus further
includes
means responsive to a continued interruption in the dc signal on
said facility for terminating a communication connection between
said subscriber unit and said apparatus.
13. The invention of claim 8 wherein said apparatus further
includes
means responsive to said control means for transmitting a signal
representing the communicating mode of said subscriber unit over
said second facility to said remote unit.
14. The invention of claim 8 wherein said providing means includes
a voice hybrid and a digital data hybrid.
15. A subscriber unit adapted to communicate in a first or a second
operating mode over a transmission facility
characterized in that
said subscriber unit includes
first means for signaling a first operating mode by allowing the
reception of a first polarity dc signal over said facility,
second means for signaling a second operating mode by allowing the
reception of a second polarity dc signal over said facility,
and
means responsive to a subscriber input for switching either said
first or second means to said facility.
16. The invention of claim 15 wherein said switching means
concurrently switches communication signals to said facility from
either a first operating mode means or a second operating mode
means.
17. The invention of claim 15 wherein said subscriber unit further
includes
means for preventing the reception of said first and second
polarity dc signal from said facility.
18. The invention of claim 15 wherein said subscriber unit further
includes
means for outputting a humanly recognizable signal from a conflict
signal received over said facility to indicate a conflict in
operating modes between said subscriber unit and a remote unit
connected over said facility to said subscriber unit.
19. In a communication system including a subscriber unit adapted
for connection over a transmission facility to a central unit, said
system operating in either an analog or digital communication mode,
a method of signaling the communication mode between said
subscriber unit and said central unit characterized by the steps
of
(a) providing at said central unit a first polarity dc signal over
said facility representing a first operating mode,
(b) interrupting at said subscriber unit said first polarity dc
signal over said facility to represent a change in the
communication mode at said subscriber unit, and
(c) providing at said central unit in response to said interrupting
step a second plarity dc signal over said facility representing a
second communication mode.
20. In a communication system including a subscriber unit connected
over a transmission facility to a central unit, said system
operating in either an analog or a digital communication mode, a
method of signaling the communication mode between said subscriber
unit and said central unit characterized by the steps of
(a) providing at said central unit a first polarity dc signal over
said facility representing the establishment of a first
communication mode,
(b) providing at said central unit a second polarity dc signal over
said facility representing the establishment of a second
communication mode, and
(c) alternating between steps (a) and (b) in response to a
predetermined interruption of the dc signal on said facility
representing a change in the communication mode at said subscriber
unit.
21. The method of claim 20 further including the steps of
establishing the connection of either said first polarity dc signal
or said second polarity dc signal to said facility in response to a
signal received over a second facility from a remote unit and
transferring control of said providing steps (a) and (b) from said
alternating step (c) to said establishing step.
22. The method of claim 20 further including the steps of
comparing the communicating mode of said facility with a
communicating mode of a second facility connected to said central
unit and
sending a conflict signal over said facility to said subscriber
unit in response to a difference in communicating modes between
said facility and said second facility as determined by said
comparing step.
23. A method for controlling the analog or digital communication
mode of a transmission facility characterized by the steps of
providing either a first polarity dc signal over said facility to
represent a first communication mode or a second polarity dc signal
over said facility to represent a second communication mode and
interchanging the dc signal coupled to said facility during the
providing step in response to a predetermined interruption in the
dc signal on said facility.
24. A method of signaling a change in the analog or digital
communication mode of a transmission facility characterized by the
steps of
(a) signaling a first communicating mode by receiving a first
polarity dc signal over said facility,
(b) signaling a second communicating mode by receiving a second
polarity dc signal over said facility, and
(c) switching between steps (a) and (b) in response to a subscriber
input.
Description
TECHNICAL FIELD
This invention relates to signaling arrangements and more
particularly to a method and apparatus for signaling the analog and
digital data operating modes of a communicating system.
BACKGROUND OF THE INVENTION
The increasing need for data communication services is being
accommodated today by using both separate data networks and by
integrating digital data communication service over existing voice
communication (telephone) networks. Typically, when both voice and
digital data communication capabilities are provided over the
existing telephone network a modem is utilized to provide the
digital data communications. In such systems, the digital data is
converted to an analog signal using the modems and no changes are
required at the local telephone central office to provide a digital
data communication channel. However, when it is desirable to
transmit high speed data as a digital signal, circuit changes are
required at both the user's location and at the local telephone
office to enable the transmission and reception of the digital
signal over the loop facility. Additionally, if it is desirable to
have a system with a capability to transmit either voice or digital
signals over the loop facility, the user must signal the operating
mode of the system to the local telephone office.
In one prior signaling method the customer signal changes in the
system operating mode to the local office by dialing special codes
and awaiting a confirmation signal from the local central office.
In such an arrangement, the operating mode signaling from the
customer to the local office exists as either frequency tones or
digital data characters. When frequency tones are used for mode
signaling a tone receiver is required at the local office.
Similarly, if digital data characters are used for mode signaling,
a digital data receiver is required at the local office. Both the
frequency tone mode signaling and digital character signaling
arrangements require using the same loop facility used for the
transmission of the voice or digital data information. To prevent
interference with the voice or digital data signals mode, signaling
must be done at a time when no voice or digital data signals are on
the loop facility. Consequently, since the system operating mode
information is not always present on the loop facility, system
memory must be utilized to remember the system operating mode.
Additionally, this memory must be accessed each time the operating
mode information is required by the telephone network. The above
tone or digital character signaling arrangements result in
additional complexity in the circuitry of a shared voice/digital
data system which share a common telephone network.
SUMMARY OF THE INVENTION
The disclosed communication system operating mode signaling method
and arrangement utilizes dc current signaling over the loop
facility connecting the customer to the local central office. This
dc current signaling arrangement provides both continuous
voice/digital data operating mode signaling and continuous
on-hook/off-hook signaling over the loop facility connecting the
customer unit to the central office unit.
In the present invention, the existence of a dc current flow over
the loop facility indicates an off-hook condition at a customer's
unit while the direction of the current flow over the facility
indicates the communicating or operating mode of the system. A
current flow in a first direction over the transmission facility
signals a system voice operating mode while a current flow in the
opposite direction signals a system digital data operating mode. A
customer or subscriber signals a change in the system operating
mode by reversing the direction of the current drain at the
customer unit. The resulting interruption of the dc current flow is
detected by the central office unit which responds by reversing the
polarity of the current source connected to the facility. The
reappearance of a dc current flow on the facility distinguishes a
customer's change in operating mode from a customer's call
termination (on-hook) condition.
In a preferred embodiment, the disclosed dc signaling arrangement
is utilized as part of an alternate voice/digital data system where
the customer can alternately switch an active call between the
voice and digital data operating modes. In such a system, the
customer (or subscriber) dials an access code and the telephone
number of the called party. The access code is utilized by the
telephone network to distinguish a voice only call from an
alternate voice/digital data call.
The initial connection between the calling and called customer is
established in the voice mode. Once the connection is established
either the calling or the called customer can change the direction
of allowed current flow on the facility to the central office to
initiate an operating mode change from the voice mode to the
digital data mode and vice versa.
Timing circuits at the central office distinguish a hit or other
inadvertent dc current interruption from a customer's valid
operating mode change. The central office responds to a valid
operating mode change current interruption by reversing the
polarity of the battery feed to the facility. If the current flow
over the customer's facility does not resume within a predetermined
time period the central office recognizes this condition as a
customer on-hook condition rather than a customer request to change
from a voice to a digital data operating mode. If a valid digital
data operating mode is established the central office makes
appropriate network changes to facilitate digital data transmission
between the customers.
At the conclusion of voice and digital data transmission either of
the customers can go on-hook interrupting the dc current flow over
their respective facilities causing the central office to release
the established connection.
BRIEF DESCRIPTION OF THE DRAWINGS
The principles of the invention will be more fully appreciated from
the illustrative embodiment shown in the drawing, in which:
FIG. 1 shows a circuit diagram of the customer apparatus of my
invention.
FIG. 2 shows a circuit diagram of the central office apparatus of
my invention.
FIG. 3 shows the association of FIG. 1 and FIG. 2.
FIG. 4 shows a typical local call connection path between
customers.
FIG. 5 shows the signaling bits utilized by my invention.
FIG. 6 shows a typical tandem call connection path between
customers.
GENERAL DESCRIPTION
With reference to FIG. 1, the apparatus located at a customer or
subscriber location is shown. The apparatus includes telephone set
SA and data set DA which are shown connected to interface unit IUA
via cables 101 and 102 respectively. Similarly, at customer
location X telephone set SX and data set DX connect to interface
unit IUX via cables 103 and 104 respectively. Interface units IUA
and IUX connect via loop transmission facilities LA and LX,
respectively, to local central office COA (shown in FIG. 2).
Interface unit IUA includes switch SW1 for switching or coupling
the signaling (dc current) means to the facility. These operating
mode signaling means (D1, D2) signal a voice operating mode or a
digital data operating mode of the system. While in the preferred
embodiment the two operating or communicating modes are
illustratively used for communicating voice or digital data
signals, it will be immediately obvious to one skilled in the art
that other types of signals can be communicated during these
operating modes. Diode D1 provides a means for signaling a voice
operating mode to central office COA by enabling a current flow
from the tip T lead to the ring R lead of loop facility LA. Diode
D2 provides a means for signaling a digital data operating mode to
central office COA by enabling a current flow from the ring R lead
to the tip T lead of loop facility LA.
With reference to FIG. 2, facilities LA and LX terminate on switch
SWA of central office COA. Channel units CUA and CUX at central
office COA also connect to switch SWA. While switch SWA shown in
FIG. 2 is preferably a No. 1/1A ESS, my invention can similarly be
implemented with any crossbar or step by step type of central
office switch which can pass dc currents. The operation of the No.
1 ESS is well known, for example see the September 1964 issue of
the Bell System Technical Journal. In the description that follows
only the operation of circuitry of the No. 1 ESS required for an
understanding of my invention will be discussed. Common control CCA
enables switch SWA, in a well-known manner, to provide a
communication connection between the loop facilities (LA and LX)
and channel units (CUA and CUX). Channel units CUA and CUX as well
as common equipment CEA are part of digital channel bank DCBA.
Channel units CUA and CUX convert the analog voice or digital data
signals from customer A and X into a digital format for
transmission over a digital carrier (e.g. T1 carrier) line T1.
While a T1 carrier is described herein, other digital carrier
systems can obviously be utilized. Carrier line T1 connects central
office COA with other central offices. Common equipment CEA of
digital channel bank CBA multiplexes the signals from channel units
CUA and CUX onto interoffice T1-carrier line T1. The operation of a
typical digital channel bank is described in U.S. Pat. No.
4,059,731 issued on Nov. 22, 1977 to Green et al which is
incorporated by reference herein. Only the operation of circuitry
of the channel unit required for an understanding of my invention
will be discussed herein.
Channel units CUA and CUX utilized in the disclosed invention
include voice hybrid VH which establishes, during a voice operating
mode, a first direction of current flow (from T and R lead) over
loop facilities LA and LX. Also included is digital data hybrid DH
which establishes, during a digital data operating mode, a
direction of current flow (from R to T lead) over loop facilities
LA and LX. Relay R1 detects interruption of loop current flow.
Relay R2 provides a means for switching between the voice hybrid VH
and digital data hybrid DH or vice versa in response to a current
interruption signal from relay R1. Loop status circuit is
responsive to a continued interruption of loop current to cause a
termination of the voice or data operating mode by appropriately
signaling common control CCA.
Common control unit CCA of FIG. 2 can be implemented as a separate
computer controller operating under program control or can be
incorporated as part of the controller for either switch SWA or
digital channel bank DCBA. In the following description common
control unit CCA controls channel units CUA and CUX, common
equipment CEA, and switch SWA to enable the various call
connections through central office COA.
DETAILED DESCRIPTION
Establishing a Voice Mode Connection
Referring to FIG. 1, assuming that telephone set SA is on-hook, no
current flows over the tip T and ring R leads of facility LA. A
telephone call is initiated when a subscriber or customer A places
telephone set SA in an off-hook condition. In the closed signaling
arrangement the on-hook to off-hook transition occurs only during a
voice operation mode. Before going off-hook at telephone set SA
customer A places interface IUA in the voice mode by setting switch
S1 to the voice position V. Switch SW1 provides a control for
customer A to switch between the voice and digital data operating
modes. Switch SW1 could be made to switch to the voice mode
automatically when customer A goes off-hook. When telephone set SA
goes off-hook, current is provided by central office COA over the
tip lead T through make switch hook contact M, switch S1, diode D1,
dial contact DL, voice circuit V1 and back to central office COA
via ring lead R. Thus, when subscriber interface IUA enables a
current from the T lead to the R lead a voice operating mode is
signaled to central office COA. While the disclosed invention is
described with reference to a dc current interruption used to
provide the on-hook and off-hook signaling (supervision), obviously
any discrete dc current change on the facility, initiated by a
subscriber, could be utilized to provide such on-hook and off-hook
signaling. Additionally, dc voltage signaling could also be
utilized in which no dc current flows over the facility.
Referring to FIG. 4, when central office COA detects current flow
from the T to R leads it performs the well-kown normal loop tests
(power cross, etc.) and connects a switching path 401 through
switch SWA between customer A and a digit receiver (DR) 420. Digit
receiver 420 provides a dial tone to customer A. These operations
are the standard well-known functions performed at central office
locations.
Customer A starts dialing (using rotary dialing or TOUCH-TONE
dialing) when dial tone is received. Customer A dials the alternate
voice/digital data V/D accesss code to indicate to central office
COA that customer A desires to send voice or digital data over loop
A. Thereafter, customer A dials the telephone number of customer B
located at a second central office COB. When customer A has
completed dialing common control CCA of central office COA seizes a
channel unit CUA of an outgoing trunk of carrier facility T1 to
central office COB. Since not all customer facilities at central
office COA are provided with the disclosed alternate voice/digital
data V/D capability central office COA must detect the V/D access
code on an originating call and then connect it to a channel unit
CUA and trunk having the V/D capability. The receiving central
office COB knows a V/D call is being received since a V/D trunk is
being utilized. Common control CCA sets up a switching connection
402 between digit transmitter (DT) 421 and channel unit CUA.
Central office COB detects a seizure of an incoming trunk and sets
up a switching connection 404 to a digit receiver (DR) 422. Digit
transmitter (DT) 421 of central office COA sends dialed digits to
receiver 422 of central office COB. When the dialed digit transfer
is complete central office COA releases switching connections 401
and 402 and establishes switching connection 403. Central office
COB releases switching connection 404 and establishes switching
connection 406 which connects a ring generator 423 to loop LB of
the called subscriber or customer B. Central office COB also sets
up connection 407 to provide an audible ringing signal back to
customer A.
When customer B answers the call at telephone set B current flows
from the T lead to the R lead of loop facility LB. When central
office COB detects this current flow it releases switching
connections 406 and 407 and establishes switching connections 405.
Thus, a voice connection path now exists from customer A over loop
A, switching connection 403, T1-carrier line or facility T1,
switching path 405 and loop LB to customer B. Customer A and
customer B can commence their voice communications.
With reference to FIG. 1, voice circuit V1 provides the means for
communicating voice signals over facility LA during the voice
operating or communicating mode. It is to be noted that switch SW1
switches the facility current through either voice circuit V1 or
digital data coupler 107. While the disclosed arrangement shows
telephone set SA powered directly from loop LA, it is contemplated
that telephone set SA could be powered at the customer or
subscriber location with appropriate switch contacts and circuitry
to provide the disclosed signaling functions.
In both loop LA to customer A and loop LB to customer B current
flows from the T lead to the R lead signaling a voice mode of
operation to channel units CUA and CUB respectively. Communication
of the voice or data operating mode information between central
offices occurs using channel units CUA and CUB. The channel units
encode the mode information as signaling bits of the T1-carrier
signal.
In the above-described connection, the channel units CUA and CUB
are in a "local" state and connect respectively to customer
interface IUA and IUB. Channel units CUA and CUB also connect to
T1-carrier facility T1. Carrier facility T1 is also referred to
herein as carrier line T1 or carrier trunk T1. In the local state
the operating mode of each channel unit (CUA or CUB) connected to a
carrier facility (T1) at an end office (originating office COA or
terminating office COB) should always follow the operating mode of
each interface (IUA or IUB) connected via the associated loop
facilities (LA or LB). To maintain this condition the channel units
CUA and CUB detect the operating mode of these respective interface
units IUA and IUB by determining the direction of current flow on
the respective loops LA and LB.
Channel units CUA and CUB inform each other of their operating mode
by encoding bit 8 of frame 6 and 12 of the outgoing T-carrier
signal on carrier trunk T1. With reference to the well-known
T-carrier frame format shown in FIG. 5, bit 8 of frame 6 of each
superframe is referred to as bit A while bit 8 of frame 12 of each
superframe is referred to as bit B. There are many possible ways to
encode the information regarding the operating mode. For purely
illustrative purposes, we will use the following convention A=0,
B=1 indicates a voice on-hook condition while A=1, B=0 indicates a
voice off-hook condition. Thus, when interface IUA is in the
on-hook condition, common control CCA places CUA in a local on-hook
condition with A=0, B=1 and voice signals encoded in a PCM format.
When IUA is in the voice off-hook condition common control CCA
places CUA in a local off-hook condition with A=1, B=0 and voice
signals from customer A are encoded in a PCM format. In the digital
operating mode, which will be discussed later, a steady value B8=0
in all frames indicates the digital control signaling mode and a
steady value B8=1 in all frames indicates the digital data mode.
Since all V/D calls are initiated in a voice mode the digital data
mode is by definition always in an off-hook condition.
As will be discussed later a conflict exists when a channel unit
(CUA) determines that its customer's interface (IUA) is in a
different (i.e., conflicting) mode than the channel unit (CUB) to
which it is connected. During such a conflict the channel unit CUA
sends either Far-End-Voice (FEV) control mode characters or
Far-End-Digital (FED) tone to its interface (IUA) to indicate the
state of the far end customers interface (IUB). A channel unit
determines a conflict by comparing its own mode (which follows the
mode of the connected IU) with the A and B bit pattern that it is
receiving over the T-carrier facility T1.
Returning to the above-described established voice connection
between customer A and customer B, each channel unit CUA and CUB
sends A=1 and B=0 to each other. With reference to channel unit CUA
of digital channel bank DCBA shown in FIG. 2, in the voice mode
relay R2 is operated over lead 211 and the make contacts of
transfer contacts R2-1 and R2-2 connect voice hybrid VH to line
201. Note, since channel units CUA and CUB are connected in a local
state connection lead TAN out of common control CCA is at ground
and hence relay R5 is not operated. Thus, the break contact of R5-3
remains closed enabling loop control 206 to control relay R2.
Analog voice signals from voice hybrid VH are sampled and pulse
amplitude modulated (PAM) by converter A/P and are transmitted as
signal TPAM to common equipment CEA. Common equipment CEA converts
the PAM signal into a pulse coded modulation PCM signal which is
then modulated as one of the channels of the T-carrier data stream
shown in FIG. 5. The previously referenced U.S. Pat. No. 4,059,731
more completely describes the operation of a channel unit in a
digital channel bank arrangement.
Returning to FIG. 2, loop status circuit 205 receives the status of
loop LA and indicates the on-hook/off-hook status over lead SL to
common control CCA. When current is flowing over line 201, the
extention of loop LA through switching machine SWA, relay R1 of
loop current monitor 200 is operative and make contact R1-1 puts a
ground potential on lead 203 to loop status circuit 205.
Additionally, loop status circuit 205 receives signal TS when a
mode transition from a voice to digital or vice versa is occuring
on loop LA. For either a ground on lead 203 or a TS signal on lead
204 loop status circuit 205 provides an off-hook signal over lead
SL to common control CCA. Otherwise loop status circuit 205
transfers the on-hook status over central lead SL to common control
CCA. Control signals DS and VS from loop control circuit 206
indicate, respectively, the digital or voice operating mode of
customer A. Since channel unit CUA is in the local state, as noted
previously, relay R5 is not operated and the break contacts of R5-1
and R5-2 remain closed. Unit 207 converts signals DS and VS into a
signal on lead V-D which indicates a voice mode and causes common
equipment CEA to encode the A=1, B=0 voice off-hook bit pattern in
the T-carrier signal on carrier facility T1. Additionally, during
the voice mode common equipment CEA accepts pulse amplitude
modulated voice signals on lead TPAM and converts them into PCM
digitized voice for transmission over carrier facility T1. Likewise
it receives PCM digitized voice which is converted to pulse
amplitude modulated voice signal and placed on lead RPAM. Note,
during the digital data mode a signal exists on lead V/D indicating
such a condition to common equipment CEA of digital channel bank
DCBA. In the digital data mode digital channel bank DCBA accepts
the PCM signal on lead T8B for transmission over carrier facility
T1 and the received PCM signals are outputted on lead R8B.
Voice signals received from customer B over facility T1 are
converted from a PCM signal to a PAM signal RPAM by common
equipment CEA. A PAM to analog converter P/A reconstructs the voice
signal for transmission through voice hybrid VH, line 201, switch
SWA to customer A via loop LA.
Trunk control 208 monitors the bit 8 pattern received on line RB8
from common equipment CEA. Lead RB8 outputs the bit 8 (A and B
bits) pattern of the data received over facility T1. If bit 8 is
constantly a 0 or a 1 the incoming bits A and B are both constantly
a 0 or 1 indicating a digital data mode and hence lead DT is at
logic 1. If the bit 8 pattern is alternating (A=0, B=1, or A=1,
B=0) indicating a voice mode then lead VT is logic 1. Conflict
circuit 209 includes relays R3 and R4 which indicate respectively
voice or digital mode conflicts. Since customer A is in a voice
mode, signal VS is logic 1, if a voice mode signal VT is also logic
1 no conflict exists and relay R3 is not operated and the make
contact of transfer contact R3-1 is open and prevents a
far-end-digital FED tone from being outputted to customer A.
However, if customer B is in a digital mode, signal VT is logic 0
and relay R3 is operated and break contact of R3-1 opens to prevent
signals from customer B via unit P/A from reaching voice hybrid VH
and customer A. Additionally, a FED tone from unit 215 is connected
by the make contact of R3-1 to voice hybrid VH and customer A. The
FED tone alerts customer A that customer B is in the digital data
mode.
Note, trunk status circuit 210 also receives the bit A and B status
over leads RA and RB and generates a trunk status signal ST for
common control CCA.
Switching Between the Voice and Digital Data Modes
Either customer A or customer B can initiate a change from the
established voice connection to a digital data connection. The
following assumes that customer A switches to the digital mode.
Referring to FIG. 1 again, customer A sets the operating mode
selection switch SW1 to the digital data position D. Thus, the
current flow from lead T through the mode switchhook contact SH,
switch SW1, diode D1, dial contact DC, voice circuit V1 to lead R
is interrupted. The presence of diode D2 in the digital position
path prevents current from flowing from the T lead to the R lead of
loop LA. While diodes D1 and D2 are illustrated as implementing
circuits to selectively enable a certain direction of current flow
over loop LA other circuits (transistors, etc.) are known to
implement such characteristics.
Referring to FIG. 2, loop current monitor 200 detects an
interruption of current on line 201 which is connected through
switch SWA to loop LA. When the current on loop LA is interrupted
relay R1 detects the interruption and releases causing make contact
of R1-1 to open and an ungrounded signal to appears on lead 203. An
interruption of current is an indication to channel unit CUA that
either (1) a mode change command is being sent (2) that customer A
has gone on-hook or (3) that a hit has occurred on loop LA. Current
monitor 200 incorporates a timer or delay 212 which waits an
appropriate length of time to insure that a hit has not occurred
before changing the state of relay R2. The combination of relay R1,
control circuit 206 and relay R2 provide a means for alternating
the connection of voice hybrid VH and digital hybrid DH which
generate the two directions of current flow over facility LA.
In response to a current interruption signal on lead 203, which
exceeds the predetermined delay 212, circuit of loop control
circuit 206 releases relay R2 and starts timer 213 which causes a
logic 1 signal TS on lead 204 for a timed interval. If the current
on loop LA reappears before timer 213 expires then the condition is
regarded as a mode change request. However, as will be discussed
later if the current on loop LA does not reappear, the condition is
an on-hook condition by customer A.
In our mode change example the TS signal into loop status circuit
205 keeps an off-hook signal condition on lead SL to common control
CCA. Thus, even though loop LA current is interrupted a termination
or on-hook signal condition from circuit 205 to common control CCA
does not result because of the timed or delayed transition signal
TS on lead 204. When relay R2 releases, the make contacts of R2-1
and R2-2 are opened and the tip to ring current generated by the
connection of voice hybrid VH to line 201 is terminated. A
connection is established between digital hybrid DH and line 201
via the break contacts of R2-1 and R2-2. Relay R2 provides a means
for connecting either the voice hybrid VH or the digital hybrid DH
to facility LA. Relay R2 is operative from control signal 211 which
is produced by control circuit 206. When digital hybrid DH is
connected a negative voltage -V is applied to the T lead and a
ground potential is applied to the R lead of line 201. The
connection of digital hybrid DH to loop facility LA enables the
generation of a current flow from the R lead to the T lead of loop
facility LA.
Thus, referring again to FIG. 1, loop LA has -V on its T lead and a
ground on its R lead. A current flow exists from lead R through
coupler 107, diode D2 and make contact M of switchhook SH to lead
T. This direction of loop current flow at subscriber interface IUA
signals a digital data operating mode to central office COA. Since
no current flows through voice circuit V1 telephone set SA is
inoperative. Data unit DA, however, is now connected to loop LA via
receive circuit 105, send circuit 106 and coupler 107.
Referring to FIG. 2, the resumption of current flow over loop LA
and hence line 201 activates relay R1 of current monitor 200. Make
contact R1-1 operates and causes a continued off-hook status signal
SL to be given from loop status circuit 205 to common control CCA.
The ground signal on lead 203 from make contact R1-1 also causes a
logic 1 signal on lead DS and a logic 0 signal on lead VS to be
generated by loop control 206. The timer 213 of loop control 206 is
also cleared by the ground on lead 203 and consequently signal TS
on lead 204 becomes logic 0. Loop control 206 signals common
control CCA via the logic 1 on lead DS that a digital mode has been
established by customer A.
Loop control 206 signals digital hybrid DH that a voice-to-digital
transition has been initiated by customer A. Thereafter, digital
hybrid DH and interface IUA go through an automatic balance
sequence. The details of this balance sequence is not necessary to
the understanding of the disclosed invention.
At channel unit CUB a conflict circuit (equivalent to 209 of
channel unit CUA) detects that customer B is in the voice mode (VS
is logic 1) and customer A is in the digital mode (VT is logic 0).
Consequently, conflict relay R3 of channel unit CUB operates
causing a FED tone to be outputted via make contact of R3-1 to
customer B. Customer B receives the FED tone over the receiver of
its handset (equivalent of HA of telephone set SA of FIG. 1).
Customer B then decides whether it wants to switch to a digital
mode.
After digital hybrid DH of channel unit CUA has been equalized it
sends FEV (far end voice) characters to customer A interface IUA. A
FEV character is sent to customer A via the make contact of R4-1
when relay R4 of conflict circuit 209 operates. Relay R4 operates
when it receives a logic 1 on lead DS while a logic 0 is on lead
DT. Lead DS being logic 1 since customer A has switched to the
digital mode and lead DT being logic 0 since customer B is still in
the voice mode. Referring to FIG. 1 again, the FEV signal is
received from coupler 107 by FEV detector 108. Detector 108 causes
lamp 109 or other indicator to turn on indicating that the far end
customer B is in the voice mode. To remove this conflict situation
customer A can wait a few moments for customer B to switch to the
digital mode or can return to the voice mode.
Assuming that customer B switches to the digital mode the
previously described operation for customer A would be repeated at
customer B interface IUB and at channel unit CUB. That is, customer
B sets switch SW1 to the digital mode causing a current
interruption over loop LB. Then channel unit CUB reverses the
battery feed to loop LB to reestablish current flow and the digital
hybrid is equalized. In a manner similar to that described for
interface IUA the hybrid of interface IUB is equalized. When
equalization is completed at both interface IUA/channel unit CUA
and interface IUB/channel unit CUB bidirectional digital
communications between customers A and B commences over loop LA,
trunk T1 and loop LB.
At channel unit CUA the digital hybrid DH, transmit data circuit
TD, and receive data circuit RD communicate digital data between
loop LA (via line 201) and common equipment CEA. Transmit data
circuit TD outputs a 8-bit PCM data word over lead T8B. Common
equipment CEA multiplexes the digital data from lead T8B into
T-carrier format, of FIG. 5, for transmission over carrier facility
T1. The digital data from common equipment CEA is demultiplexed
into a 8-bit PCM data word on lead R8B which is coupled to receive
data circuit RD.
It is to be noted that current monitor 200 of channel unit CUA
continuously monitors current flow over loop LA. The direction of
current flow on loop LA gives a continous indication of the digital
operating mode of customer A. Likewise, the direction of current
flow over loop LB gives channel unit CUB a continous indication of
the operating mode of customer B. Additionally, channel unit CUA
continuously knows the operating mode status of customer B by
virtue of the continous pattern of signaling bits A and B received
over carrier facility T1 from channel unit CUB. Similarly channel
unit CUB continuously knows the operating mode of customer A by
virtue of the continous pattern of signaling bits A and B. Thus,
the operating mode status of both customer A and customer B is
constantly available from the above signal conditions on loop LA,
loop LB and carrier facility T1.
After digital data communication between customer A and customer B
are completed either customer can hang up or return to the voice
operating mode.
Off-Hook to On-Hook Transition
With reference again to FIG. 1, assume that customer A desires to
go on-hook and terminate the digital communication mode. When
customer A goes on-hook make contact M of switchhook SH is opened
preventing current flow in loop LA. Break contact B of switchhook
SH closes connecting ringer R1 across lead T and lead R. Ringer R1,
however, does not conduct dc current and hence no current flows in
loop LA.
With reference to FIG. 2, relay R1 of current monitor 200 is
released by the interruption of current flow. When make contact
R1-1 opens, timer 213 in loop control circuit 206 generates a
transition signal TS on lead 204 to enable loop status circuit 205
to temporarily maintain an off-hook signal SL to common control
CCA. Loop control circuit 206 operates relay R2 which disconnects
digital hybrid DH and reconnects voice hybrid VH resulting in a
battery reversal across the T and R lead of line 201 and hence loop
LA. Since customer A has gone on-hook at telephone set SA current
flow remains interrupted on loop LA when battery is reversed. After
a continued current interruption on loop LA lasting a predetermined
period of time, timer 213 of loop control circit 206 times out and
transition signal TS disappears from line 204. In response to the
continued loop current interruption lead SL of loop status circuit
205 gives mode terminating on-hook signal to common control CCA
since lead 203 is not grounded and signal TS on lead 204 is not
present. Common control CCA causes a voice on-hook condition (bit
A=0, B=1) to be sent from channel unit CUA to channel unit CUB as
part of the T-carrier signal on carrier facility T1. Referring to
FIG. 4, the bit A=0, B=1 condition causes central office COA and
central office COB to disconnect existing switching connections 403
and 405 respectively, thus terminating the connection between
customers A and B. Channel unit CUA reverts to the voice mode when
it is idle (on-hook).
Switching Between the Digital and Voice Modes
Assuming that customer A and customer B had an established digital
connection between them, either customer A or customer B can
initiate a return to the voice mode. With reference to FIG. 1 and
assuming that customer A initiated the change from a digital to a
voice mode, the following events take place. Customer A operates
switch SW1 of interface IUA to the voice position V. The existing
current flow from the R lead through coupler 107, diode D2, and
switch SW1, make contact M to the T lead of loop LA is
interrupted.
Referring to FIG. 2, current monitor 200 detects the current
interruption and releases relay R1. Again timer 213 of loop control
circuit 206 outputs a transition signal TS to loop status circuit
205. Loop status circuit 205 maintains an off-hook signal SL to
common control CCA. Loop control circuit 206 also operates mode
switching relay R2. The operation of relay R2 disconnects data
hybrid DH and connects voice hybrid VH causing a battery reversal
on the T and R leads of loop LA via line 201 and switch SWA. When
current flow resumes on loop LA in a direction from lead T to lead
R relay R1 operates. When relay R1 operates make contact R1-1
closes and timer 213 of loop control circuit 206 is reset. Make
contact R1-1 places a ground on line 203 which enables loop status
circuit 205 to maintain an off-hook signal to common control CCA.
Loop control circuit 206 outputs a logic 1 signal on lead VS and a
logic 0 signal on lead DS signifying a voice operating mode for
customer A. Common control CCA receives signals on leads DS and VS
and causes the voice mode A=1, B=0 bit pattern to be generated by
common equipment CEA. Conflict circuit 209 receives the logic 1 on
lead VS and releases relay R3 if a logic 1 signal exists on lead
VT. A logic 1 signal would exist on lead VT only if customer B has
switched to the voice mode and hence a A=1, B=0 bit 8 pattern is
received on lead RB8 from the T-carrier signal received over
carrier facility T1 from customer B.
If customer B has not switched to the voice mode lead VT has a
logic 0 signal and relay R3 of conflict circuit 205 is operated.
Consequently, make contact of R3-1 permits a FED (far end digital)
tone to be outputted through voice hybrid VH to customer A. Thus,
customer A knows of the status of his request to change from the
digital to the voice operating mode. When customer B switches to
the voice mode relay R3 releases and make contact R3-1 connects the
signals received from customer B, via lead RPAM and converter P/A,
to customer A.
At channel unit CUB (not shown) the transition of customer A from a
digital mode to a voice mode results in a FEV (far end voice)
control mode character being sent to customer B until customer B
changes to the voice mode. A FEV character results because channel
unit CUB is in the digital mode while channel unit CUA is in the
voice mode. At interface IUB the FEV character results in a visual
output FEV signal to customer B. When customer B switches to the
voice mode the transition sequence of events previously described
for customer A occur in the corresponding interface unit IUB and
channel unit CUB of
After customer B has made the transition from the digital to the
voice mode channel unit CUA and channel unit CUB signal
respectively customer A and customer B indicating that a voice
connection is complete. Note that equalization or balancing of the
voice hybrids is not required on transactions to the voice mode.
Voice communication between customer A and customer B ensues.
During the voice operating mode the direction of current flow from
the T lead to the R lead of loop LA gives channel unit CUA a
continuous indication of the operating status of customer A via
interface IUA. Channel unit CUA also continuously knows the
operating mode status of customer B by virtue of the continuous
pattern of signaling bits A=1, B=0 received over the carrier
facility T1 from channel unit CUB. In a similar manner channel unit
CUB has a continuous status indication of customer B and customer
A. At the completion of voice communications either customer A or
customer B can request a change to the digital operating mode or
hang-up.
The above paragraphs have described the operation of my invention
for signaling the voice and digital operating modes between
customers having different local central offices. With references
to FIGS. 1, 2 and 4, the connection between a customer A and a
customer X which share the same local central office proceeds as
previously described except that all the connections described at
central office COB also occur in a similar manner at central office
COA. As shown in FIG. 4, the connection between channel unit CUA
and channel unit CUX would be through T-carrier loop-back
connection 424 at central office COA.
Channel Units in a Tandem Connection
FIG. 6 illustrates a connection between a customer A at central
office COA and a customer D at central office COD, involving two
tandem switching offices COB and COC. For illustrative purposes, it
is assumed that COB is a No. 1A ESS and that COC is a No. 4 ESS.
The end offices COA and COB are again assumed to be No. 1A ESS
switches.
Customer A is connected to office COA via interface unit IUA and
customer D is connected to office COD via interface unit IUD. As
described before, channel units CUA and CUD are in a "local" state
since they are connected directly to the customer's two-wire
facilities (loops LA and LB). However, in the tandem switch of COB,
channel units CUB 1 and CUB 2 are in a "tandem" state since they
are connected by the switching network SWB.
With reference to FIG. 2, assume that CUA is connected in a tandem
state. Common control CCA provides this "tandem" indication to CUA.
For illustrative purposes, FIG. 2 shows this indication as the
presence of a voltage on lead TAN, which operates relay R5. This
causes CUA to be in a tandem state. In this state, circuit 206 no
longer controls whether hybrid DH or hybrid VH is connected to the
two-wire port. This mode control is now achieved by circuit 208
(via relay contacts R5-3) which receives the analog/digital mode
indication on RB8 from the T1 line, and operates or releases relay
R2. In other words, when a channel unit is in a "tandem" state, it
connects hybrid DH or hybrid VH and the associated DC polarities on
the T and R leads of the two-wire port, as dictated by the mode
indication on the incoming T1 bit stream.
In addition, when in a tandem state, the channel unit uses relay R1
as a conflict indicator. When two connected CU's are in the same
mode, they present the same hybrids and the same polarities on the
T and R leads. Consequently, no current flows in relay R1 of either
channel unit. When two connected channel units are in opposite
modes, they present different hybrids, hence opposite polarities on
the T and R leads. This causes relay R1 in both units to operate.
Hence, when in a tandem state, the operated state of R1 indicates a
mode conflict, while the released state of R1 indicates a mode
agreement with the connected channel unit.
When in a tandem state, a channel unit has to indicate the state of
the connected channel unit to its outgoing T1 line, by presenting
the proper voice/digital indication on lead V-D of FIG. 2. Remember
that R5 is operated. When R1 is released, the outputs of trunk
control circuit 208 are connectes straight through to the inputs of
circuit 207, via the break contacts R1-2. As a result, when, for
example, circuit 208 detects a digital mode indication on the
incoming T1 line and CUA is therefore in the digital mode, the
output of circuit 207 also indicates the digital mode, indicating
that, because no conflict is detected, the connected channel unit
is also in the digital mode. However, if R1 is operated, the
outputs of circuit 208 are cross-connected to the inputs of circuit
207 via the make contacts R1-2. In this condition, a "digital"
indication received on 208 results in a "voice" indication at
output V-D of circuit 207. In this manner, the channel unit in a
tandem connection pass the mode indication of the connected channel
unit to the outgoing T1 bit stream.
Finally, note that, when in the tandem state, R5 is operated and
its contacts R5-4 and R5-5 prevent operation of both relays R3 and
R4. No FED tone or FEV data pattern is even transmitted by a
channel unit when in the tandem state. These indications are
provided, when necessary, by channel units CUA and CUD of FIG. 6,
that are directly connected to the subscribers, and are therefore
in a local state.
With reference to FIG. 6, the establishing of a call between
customer A of central office COA and customer D of central office
COD proceeds in the following manner. As previously described a
call is initiated when customer A goes off-hook at station SA.
Central office COA detects the tip to ring loop current (since all
voice/digital calls are established in a voice mode) as previously
described. A digit receiver is connected to and supplies dial tone
to customer A over loop LA. Customer A dials the voice/digital data
operating mode access code and the telephone number of customer D.
Note this dialing can obviously be manually via a keypad or
automatically if interface unit IUA is so equipped. Central office
COA collects the digits in the normal manner, determines that a
voice/digital data call (V/D) is being made, and selects an
outgoing voice/digital data (V/D) trunk on carrier facility T1. As
noted previously, central office COA only selects V/D trunks to the
other central offices once a V/D call is requested. Hence, the
other central offices know a V/D call is being received since a V/D
trunk is being utilized.
With reference to FIG. 6, the selected facility involves channel
unit CUA in office COA and channel unit CUB 1 in office COB. Since
both COA and COB are assumed to be No. 1A ESS offices, the
inter-office signaling can be implemented either on a per-trunk
basis (using MF or any other signaling method), or by common
channel inter-office signaling CCIS. Office COA signals the called
number to office COB. When sufficient information on the called
party has been received by COB, a VID facility to office COC is
seized and office COB signals the called number to office COC.
Since office COC is assumed to be a No. 4 ESS, common channel
inter-office signaling (CCIS) is used between COB and COC. After
receipt of the called number, COC seizes a VID facility to office
COD and sends the called number to COD using CCIS signaling. The
call arrives at COD on a facility that includes channel unit CUD.
Office COD establishes a connection between CUD and an audible tone
circuit, and between called customer B a ringing circuit. Customer
B's telephone rings and customer A hears the audible ringing tone.
After customer B answers, COD connects IUD of the called customer
to channel unit CUD. At this point in the call, CUA is in a local
state and in the voice mode, since customer A is in the voice mode.
Channel unit CUD is likewise in a local state and in the voice mode
because customer B is in the voice mode. At office COB channel
units CUB 1 and CUB 2 are in a tandem state. CUB 1 is in the voice
mode because it receives the voice mode indication from CUA.
Likewise CUB 2 is in the voice mode because it receives the voice
mode indication from CUD via office COC.
Let us now examine the mode indications as they traverse the No. 4
ESS (office COC). On the channel of facility T2 that is used for
the connection, office COB sends the voice off-hook condition (A=1,
B=0). Remember that the A and B bits are the least-significant
channel bits occurring in transmission frames 6 and 12
respectively. Since the incoming frame on facility T2 and the
outgoing frame on facility T3 are not necessarily equal, the A and
B bits received on T2 are not necessarily A and B bits (i.e. bits
occurring in frames 6 and 12) on facility T3.
Referring to FIG. 2, assume that CUA represents CUD of FIG. 6.
Trunk control circuit 208 has to determine the mode from bits 8 (on
lead RB8) of the communication channel, but does not know which of
these bits are really A and B bits. The algorithm for detecting the
mode by examining bits 8 is simple. Remember that digital mode
indications have all bits 8 either consistently "1" or constantly
"0", while in voice on and off hook modes, bits A and B are opposed
(0,1 or 1,0), while the other ten occurances of bit 8 during a
super frame represent the least significant bit of the pcm encoded
voice sample. This means that in the voice mode, any consecutive
group of at least 12 bits 8 must contain at least one "1" and at
least one "0" since it must contain an A and a B bit. However, in
the data mode, such a group can consist of all "1"s or all "0"s.
Those skilled in the art may utilize various embodiments of this
basic algorithm to implement the function of circuit 208.
For reasons explained above circuit 210 of channel unit CUA in FIG.
2 when this CU is associated with a facility that terminates on a
No. 4 ESS cannot function. Its output ST, which in channel units
that are connected to facilities terminating on a No. 1A ESS
indicates the on-off hook condition at the distant end, is ignored
by common control CCA of office COD. Instead, common channel
interoffice signaling is used between offices COD and COC (and
similarly, between offices COC and COB) to convey such conditions
as answer (off-hook) and disconnect (on-hook).
The V/D call has now been completed in the voice mode between
customer A and customer D. In this condition, interface unit IUA
and channel unit CUA permit current flow from the tip lead T to the
ring lead R. Channel unit CUB 1 and channel unit CUB 2 both have
the tip lead T at a ground potential at the ring lead R at -48 V.
Hence, no current flows between channel unit CUB 2 and channel unit
CUB 1 (i.e., no conflict). Channel unit CUD and interface unit IUD
permit current flow from the tip lead T to the ring lead R.
At this time, normal voice conversation takes place between
customer A and customer D. As noted previously, the voice signal is
transmitted in PCM format on the T-carrier facilities and through
the No. 4 ESS located at central office COC. The PCM signal is
converted to an analog form for transmission through the No. 1/1A
ESS networks and loops associated with central offices COA, COB and
COD. At the outgoing 4-wire side of channel units CUA, CUB 1, CUB 2
and CUD the A and B bits have the values 1 and 0, respectively,
indicating voice off-hook mode.
Assuming that customer A wishes to initiate a switch to the digital
mode, the current on loop LA is interrupted to interface unit IUA.
Channel unit CUA detects the loss of loop current on loop LA and
reverses the battery and ground connects to loop LA, reestablishing
current flow to interface unit IUA. As previously described, no
reappearance of current on loop LA indicates that customer A has
gone on-hook. Assuming current on loop LA resumes, channel unit CUA
is placed in the digital mode. At this time channel unit CUA and
interface unit IUA immediately begin balancing their hybrids as
previously described. During the balancing process, channel unit
CUA continuously transmits via lead T8B a "transition in progress"
(TIP) control mode characters (in which bit 8=0) on the outgoing
T-carrier line T1 and monitors only bit 8 via lead RB8 on the
incoming T-carrier line T1. At the end of the balancing process,
channel unit CUA detects a mode conflict since it is in the digital
mode and the incoming T-carrier signal on carrier facility T1
indicates (via A/B bits which are 1/0) that interface unit IUD is
in the voice mode. Accordingly, channel unit CUA continuously
generates and transmits "far end voice" (FEV) control mode
characters to interface unit IUA. In the reverse direction, channel
unit CUA simply repeats the signal received from interface unit IUA
on to the outgoing carrier facility T1. Interface unit IUA
transmits "control mode idle" (CMI) characters at this time to
channel unit CUA.
At the initiating of the balancing process, the bit 8 pattern on
carrier facility T1 directed to central office B has changed. The
previous bit 8 pattern A/B=1/0 (off-hook voice) pattern changed to
a constant 0 in bit 8 since the TIP and CMI characters, being
control mode, have bit 8 set to 0. Since channel unit CUB 1 is in
off-hook state, it is able to pass this mode change information to
channel unit CUB 2. This is done by reversing the battery and
ground connections to the T and R leads 601, causing loop current
to flow through the No. 1/1A ESS networks at COB to channel unit
CUB 2. Channel unit CUB 2 detects the current flow on the T and R
leads (2-wire side) and thus detects that interface unit IUA is in
the digital mode. Channel unit CUB 2 forwards this information to
interface unit IUD by generating the transmitting TIP characters on
trunk T2.
At this time, channel unit CUB 1 is receiving bit 8=0 (digital data
mode) over trunk T1 and is transmitting bits A/B=1/0 (voice mode)
over trunk T1. Channel unit CUB 1 has lead T=-48 V and lead R at
ground. Channel unit CUB 2 has lead T at ground and lead R=-48 V.
Channel unit CUB 2 is receiving bits A/B=1/0 and is outputting bit
8=0. The T-carrier bit stream on carrier facility T2 passes through
the No. 4 ESS at central office COC unchanged. However, since No. 4
ESS is a time division multiplexing machine, the bytes or words may
not be in the same position in the superframe. Thus, the concept of
A and B bits is meaningless when applied to the bit stream leaving
a No. 4 ESS. However, techniques are known to recover the necessary
information from bit 8 alone without knowing the specific identify
of A and B bits.
When channel unit COD detects that bit 8 is constantly 0 it signals
interface unit IUD that interface unit IUA is in the digital mode.
This is done by generating "far end digital" tone (FED) and sending
it to interface unit IUD over loop LD. Interface unit IUD detects
this tone and notifies customer D via a lamp (109 of FIG. 1) or
tone signal (not shown). At this time, the two interface units are
in conflicting modes, and each customer A and D is aware of the
other's mode. The two interface units IUA and IUD remain in this
configuration as long as customer A and D are ordering them to be
in opposite modes.
To complete the switch to the digital mode, customer D commands or
switches interface unit IUD to the digital mode. Interface unit IUD
reverses the permitted direction of loop current flow on loop LD.
Channel unit CUD detects the loss of loop current, reverses battery
feed, and detects the reestablishment of current on loop LD.
Interface unit IUD and channel unit CUD are now in the digital mode
and initiate the balancing and the respective data hybrids.
Channel unit CUD simultaneously transmits TIP characters (bit
8=constantly 0) on facility T3. These characters are passed through
central office COC to facility T2 and detected by channel unit CUB
2 in office COB. Since this channel unit is in a sender state, it
obeys the digital mode indication received on T2. Since CUB 1 is
already in the digital mode, the transition of CUB 2 to the digital
mode causes the disappearance of conflict indicators in both CUB 1
and CUB 2. Therefore CUB 1 transmits a digital mode indication to
CUA of central office COA. This channel unit, which is in a local
state and is therefore in a digital mode, following the command
from cutomer A, had been sending far-end voice (FEV) characters to
customer A because it was recognizing a conflict between the
digital mode of customer A and the voice mode indication arriving
on facility T1. Since CUB 1 now starts sending a digital mode
indication to CUA, this channel unit stops sending FEV characters
to customer A and repeats the digital patterns received on T1 to
the customer. At this point, exchange of 56 kb/s data between
customers A and B can begin.
When a customer wishes to return to the voice mode an appropriate
action is taken at the interface unit. Assume in this case that
customer D makes the decision to switch to the voice mode.
Interface unit IUD stops sending data, and reverses the permitted
direction of current flow on loop LD, and switches the station set
SD on to loop LD. Channel unit CUD detects the loss of loop
current, reverses its battery feed to loop LD, thus reestablishing
loop current. Channel unit CUD switches to the voice mode by
switching its voice hybrids onto loop LD and causing the
transmission of PCM encoded voice over carrier facility T3 with A/B
bits encoded as 1/0. Since the incoming T-carrier bit stream on
carrier facility T3 indicates that interface unit IUA is in the
digital mode channel unit CUD detects the incompatibility of the
modes between interface unit IUA and interface unit IUD. Being in
the voice mode itself, channel unit CUD therefore generates the
"far end digital" tone (FED) and sends it to interface unit IUD,
causing interface unit IUD to indicate to its customer that the far
end is in the digital mode.
The PCM encoded voice signal from channel unit CUD propagates
through central office COC to channel unit CUB 2. Once channel unit
CUB 2 has detected the change of the incoming bit 8 pattern, it
switches into the voice mode. This switch involves connecting the
voice hybrid on the 2-wire side of the channel unit, reversing the
battery connection and interpreting the incoming signal from
T-carrier facility T2 as PCM encoded voice. The battery reversal at
channel unit CUB 2 causes a loop current to flow through the No.
1/1A ESS network at COB to channel unit CUB 1.
This current indicates to channel unit CUB 2 that interface unit
IUA is in the opposite mode to interface unit IUD. Since channel
unit CUB 2 knows that interface unit IUD is in the voice mode, due
to the bit 8 pattern incoming on carrier facility T2, it determines
that interface unit IUA remains in the digital mode. Thus, channel
unit CUB 2 causes transmission of a PCM voice silent code on
carrier facility T2.
Meanwhile, channel unit CUB 1 also detects the flow of loop
current. Channel unit CUB 1 switches to the voice mode and causes
the encoding of the incoming voice signal on its 2-wire side as PCM
with A/B bits added. This PCM signal is sent out on carrier
facility T1. Since the signal incoming to channel unit CUB 1 on
carrier facility T1 continues to indicate that interface unit IUA
is in the digital mode, the incoming digital signal from carrier
facility T1 is discarded after checking its bit 8 pattern. Channel
unit CUB 1 transmits a voice silent signal on the 2-wire facility
to channel unit CUB 2.
Channel unit CUA detects the changed bit 8 pattern received on
carrier facility T1. Since channel unit CUA is in the digital mode,
it generates a digital character (FEV) to inform interface unit IUA
that interface unit IUD is in the voice mode. Channel unit CUA
generates this signal and sends it on loop LA to interface unit IUA
while continuing to pass data received from interface unit IUA to
trunk T1. Interface unit IUA detects the incoming FEV characters
and signals its customer that the distant interface unit is in the
voice mode.
At this time the two interface units are in opposite modes but both
are aware of that fact. To complete the switch to the voice mode,
customer A switches interface unit IUA to the voice mode. When this
is completed, current is interrupted on loop LA. Channel unit CUA
detects the loss of loop current, reverses loop battery feed and
switches to the voice mode. Once in the voice mode, channel unit A
begins encoding the incoming voice from loop LA (with A/B bits
added) and decoding the incoming T-carrier signal from trunk T1
into voice signals which are sent out on loop LA.
Channel unit CUA transmits the encoded voice over trunk T1 towards
channel unit CUB 1. When channel unit CUB 1 detects the changed bit
8 pattern, it reverses its battery feed to its 2-wire port. This
causes loop current to disappear, indicating that mode
compatibility has been achieved. Channel unit CUB 1 now becomes
transparent to voice signals in both directions, doing the normal
PAM encoding/decoding function. The loss of loop current flow has
the same effect on channel unit CUB 2 causing it to become
transparent to voice signals.
Channel unit CUD also detects the return of the bit 8 pattern to
that associated with A/B bit signaling. Since it is already in the
voice mode, compatibility has been achieved. Therefore, channel
unit CUD stops sending a FED tone and allows voice transmission in
both directions. Normal voice transmission may now take place
between customer A and customer D.
What has been described is merely illustrative of my invention.
While the description was directed to a voice/data communication
system the invention can be arranged to signal any two operating
modes of a digital or analog communication system. Those skilled in
the art may advantageously utilize the concepts taught herein to
implement other embodiments providing similar functions without
deviating from the scope or spirit of the disclosed invention.
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